Efficient electrically-isolated power circuits with application to light sources

ABSTRACT

Solid state light sources are compatible with AC phase-cut dimmers. The light sources may have switching mode power supplies having primary and secondary sides that are in first and second circuit parts that are electrically isolated from one another. Information regarding a waveform of input electrical power is extracted in the first circuit part and passed to a controller in the second circuit part by way of a galvanic isolator. Additional isolated paths may be provided to provide bi-directional exchange of information between the first and second circuit parts and/or to provide for the exchange of additional information relevant to control. The signal path from the first side to the second side may have a low latency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/481,898 filed 28 May 2012, which is a continuation of U.S. patentapplication Ser. No. 12/912,576 filed 26 Oct. 2010, which claims thebenefit under 35 U.S.C. §119 of the following United States patentapplications, all of which are hereby incorporated herein by reference:Application No. 61/279,750 filed 26 Oct. 2009; Application No.61/395,589 filed 17 May 2010; and, Application No. 61/363,161 filed 9Jul. 2010.

TECHNICAL FIELD

The invention relates to lighting. Some embodiments provide efficientLED light systems that may be controlled using AC phase-cut dimmers.

BACKGROUND

Conventional lighting such as incandescent lamps and fluorescent lampsare relatively inefficient. A significant proportion of the electricalpower supplied to conventional lighting fixtures is converted into heatinstead of light.

Solid-state light sources such as light-emitting diodes (‘LEDs’) canconvert electrical energy into light much more efficiently thanincandescent or fluorescent bulbs. LEDs having high power andreliability suitable for use in architectural lighting applications arenow available.

There is a general desire for light sources that can be dimmed. Manybuildings are wired with AC phase-cut dimmers. Such dimmers are capableof dimming incandescent lamps by reducing the power delivered to thelamps. This is done by cutting off a portion of the AC waveform. Mostsolid-state lighting circuits are not well suited to being controlled byAC phase cut dimmers. While solid-state lighting systems can be designedto work with different control technologies, there is a very largeinstalled base of AC phase cut dimmers. There is a need formore-efficient solid-state lighting systems that can be dimmed by ACphase-cut dimmers.

Solid state lighting systems have the advantage of improved energyefficiency. Further, a solid-state lighting system may be designed toprovide control over power factor.

Solid state lighting systems have the additional advantage that thelight emitters are powered at low voltages. Low voltage electrical poweris safe. Low-voltage components do not require the same safetycertifications that are required for lighting systems that use highervoltages such as the 110 volts or higher AC voltages typically used inNorth America to power incandescent and fluorescent lights. Thereremains a need for solid-state lighting systems that can be powered byhigher AC voltages (such as household AC current) while ensuring thesafety of users.

SUMMARY OF THE INVENTION

One aspect of the invention provides solid-state light sources. Thelight sources comprise a power input; a rectifier connected to rectifyan AC waveform presented at the power input; and a switching mode powersupply having a primary side and a secondary side. The secondary side iselectrically isolated from the primary side. In this disclosure, theterms “electrically isolated” and “galvanically isolated” when appliedto two components, terminals, circuit parts or the like mean that thereis no conduction path by which electrons can flow directly between thecomponents, terminals, circuit parts or the like.

The primary side is connected to receive electrical power rectified bythe rectifier. A solid-state light emitter (for example, a LED, OLED, orthe like) is connected to receive electrical power from the secondaryside of the switching mode power supply. A control is connected tocontrol a current being drawn by the solid-state light emitter. Thecontrol is connected to receive a signal from a monitoring circuit. Thesignal may, for example, comprise a DC voltage or current, an AC voltageor current, a series of pulses or another information-carrying signal.The monitoring circuit is connected to receive an output of therectifier and is configured to generate a signal indicative of at leastone characteristic of the output of the rectifier. The light sourcescomprise a galvanic isolator connected to carry the signal from themonitoring circuit to the control.

With this construction all of the secondary side of the switching modepower supply, the control and the solid-state light emitter can begalvanically isolated from the monitoring circuit and the primary sideof the switching mode power supply.

Further aspects of the invention and features of specific embodiments ofthe invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate non-limiting example embodiments ofthe invention.

FIG. 1 is a schematic block diagram showing a light source according toan example embodiment of the invention; and FIG. 1A illustrates ameasurement of the phase angle.

FIG. 2 is a schematic diagram illustrating an example AC filter of atype that may be used in the light source of FIG. 1.

FIG. 3 is a schematic diagram illustrating an example circuit that maybe used for extracting and carrying phase angle information to anisolated secondary side of a lighting system; and FIG. 3A illustratesthe waveform of the sample circuit.

FIG. 4 is a schematic diagram illustrating an optional circuit forapplying a non-linear transformation to a signal carried by an isolator.

FIG. 5 is a block diagram illustrating a light source according to analternative embodiment of the invention.

FIG. 6 is a block diagram illustrating a light source according toanother alternative embodiment of the invention.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

FIG. 1 shows a light source 20. Light source 20 is driven by electricalpower supplied by an AC supply 12. For example, the AC supply 12 maycomprise a supply of standard household AC current. AC supply may have avoltage in excess of 100 volts. For example, AC supply 12 may supplyelectrical current at a voltage of 120 volts, 240 volts, 277 volts, 575volts or some other suitable voltage.

A phase-cut dimmer 14 is provided in the circuit to which light source20 is connected. Dimmer 14 truncates a variable amount Y from eachhalf-cycle of the AC waveform as shown in FIG. 1A. Depending upon itsconstruction, dimmer 14 may cut the leading or trailing edges of the ACwaveform.

Light source 20 comprises an AC filter 22. AC filter 22 is configured toremove high-frequency electrical noise from the incoming AC power.

The filtered AC power is rectified by rectifier 24. Rectifier 24 isillustrated as being a full-wave bridge but may have other suitableconfigurations. It is generally desirable that rectifier 24 providefull-wave rectification although this is not mandatory in allembodiments.

The output from rectifier 24 is connected to power a switching modepower supply (‘SMPS’) 26. SMPS 26 comprises a primary side 26A and asecondary side 26B. Primary side 26A and secondary side 26B areelectrically isolated from one another. That is, there is no path bywhich electrons can flow directly from the inputs of primary side 26A tothe outputs of secondary side 26B. Primary side 26A and secondary side26B are characterized by different ground potentials in preferredembodiments.

Power filtering and conditioning are optionally provided byfilter/conditioner 25. Filter/conditioner 25 may, for example, comprisefurther filters, capacitors, regulators which take the rectified powerfrom rectifier 24 and provide DC power to SMPS 26.

Outputs of SMPS 26 are connected to supply electrical current at anappropriate voltage to a solid-state light emitter 28. Solid-state lightemitter 28 may, for example, comprise an LED light emitter comprisingone or more LEDs 29. In some embodiments, light emitter 28 comprises aplurality of LEDs 29 and a switching matrix (not shown in FIG. 1) thatpermits the LEDs 29 to be interconnected in various ways. The switchingmatrix may, for example, be operative to vary the number of LEDs thatare connected to emit light at a given time.

In the illustrated embodiment, light source 20 comprises one or morecircuits for monitoring characteristics of the AC waveform beingdelivered to light source 20. FIG. 1 shows a phase angle detector 30that extracts from the rectified AC signal a characteristic thatindicates the phase angle (e.g. measure of the parameter Y indicated inFIG. 1A) at which dimmer 14 is currently cutting off the leading and/ortrailing edges of each half-cycle of the AC waveform being provided tolight source 20. A signal 31 is carried from phase angle detector 30 toa controller 34 by way of an isolator 32 comprising galvanicallyisolated first and second sides 32A and 32B.

One or more additional or alternative monitoring circuits 38 may beprovided to monitor other aspects of the AC waveform being delivered tolight source 20. Monitoring circuit 38 may, for example, monitor one ormore of: RMS voltage; RMS power; peak voltage; the timing of AChalf-cycles; the phase shift between peak voltage and peak current (e.g.a measure of power factor); the envelope of the AC waveform; or thelike. Circuit 38 generates a signal 39 that is carried to controller 34by way of isolator 40.

Controller 34 controls one or more of secondary side 26B of SMPS 26 andsolid-state light source 28 based at least in part on signals receivedfrom circuits 30 and/or 38. In the embodiment illustrated in FIG. 1,controller 35 provides control inputs 35 to secondary side 26B andcontrol inputs 37 to solid-state light source 28.

From the foregoing description it can be seen that light source 20comprises a first part 20A in which line voltages may be present and asecond part 20B that is galvanically isolated from first part 20A.Advantageously, all parts of light source 20 that are accessible to auser may belong to second part 20B. First part 20A and second part 20Bmay have different ground potentials. Another advantage of the lightsource illustrated in FIG. 1 is that control 34 is isolated from theelectrical environment of first part 20A, which may be electricallynoisy, by isolators 32 and 40 and the electrical isolation providedbetween primary and secondary parts 26A and 26B of SMPS 26. This can bebeneficial especially in cases where control 34 is of a type that issusceptible to being damaged or being caused to operate improperly byelectrical noise.

FIG. 2 shows one possible example circuit that may be used as an ACfilter 22 in an embodiment like that of FIG. 1. A wide range of otherfilter arrangements may be used for filter 22. Filter 22 as shown inFIG. 2 comprises a first inductor L1, a second inductor L2 andcapacitors C1 and C2. In a non-limiting example embodiment, thesecomponents have the values: L1=470 μH, L2=150 μH, and C1=C2=15 nF. Anyhigh frequency electrical noise present in the AC signal at the input oflight source 20 is blocked by L1 and L2 and shunted by C1 and C2.

FIG. 3 shows an example circuit that may be used as a phase angledetector in a light source like that shown in FIG. 1. Circuit 30 may beconnected directly to the output of rectifier 24. The waveform acrossthe positive and negative inputs 31A and 31B of circuit 30 may, forexample, be as shown in FIG. 3A. The series connected circuit 33 made upof R1, R2 D1 and D2 serves as a voltage-to-current converter. Thecurrent through circuit 33 varies as the voltage across inputs 31A and31B. In a non-limiting example embodiment, R1=39 kΩ, R2=3 kΩ, and D1 andD2 are type 1N4148 diodes which each provide a forward voltage drop of1.0 volt.

The voltage dropped across R2 is applied to the inputs of optoisolator32. By way of non-limiting example, optoisolator 32 may comprise a typeCMY17-4 optoisolator as available from various manufacturers includingAgilent Technologies, Inc. of Santa Clara Calif.

Suitable circuitry is provided to extract and pass on the signal thathas passed through the optoisolator. In the illustrated embodiment, theoutput from optoisolator 32 is applied to the base of transistor Q1which acts as an amplifier to produce a voltage signal at output 31Cthat carries information regarding the phase angle at which dimmer 14 iscutting off the waveform of the AC power being supplied to light source20. In a non-limiting example embodiment, R3=81 kΩ, R4=5 kΩ, C3=C4=0.1μF and Q1 is a type MJD 340TF NPN transistor available from varioussources including Fairchild Semiconductor Corporation of San Jose,Calif.

Amplifying the output of isolator 32 is optional. Amplifying the outputof isolator 32 can provide an output in the form of pulses that are morenearly rectangular than the pulses at the input of isolator 32. In someembodiments, analysis of the signal by control 34 is facilitated byhaving larger-amplitude, more nearly rectangular, pulses. In someembodiments the signal received at the output of optoisolator 32 isprocessed using logic circuits, such as a logic inverter circuit thatprovide a logic level output signal.

Optionally circuitry that receives the output from optoisolator 32comprises a filter configured to remove or attenuate electrical noisethat may be present in the received signal.

Control 34 may take any of a variety of forms. By way of example,control 34 may comprise a programmed data processor, analog circuitry,combinations thereof, or the like. In preferred embodiments, control 34controls the magnitude of a continuous DC electrical current deliveredto drive light-emitters 29. Such DC control is in contrast to thepulse-width modulation (PWM) control often applied to dim LEDs.

Where control 34 comprises a data processor, the signal received fromisolator 32 may be processed to determine a desired dimming level and anoutput signal may be determined for the dimming level by performing acalculation based on the dimming level and stored parameters relating tothe performance of light emitters 29, looking up an output signal in alookup table, or the like. Optionally, the signal from isolator 32 issubjected to analog domain processing before it is provided to control34. For example the signal received at isolator 32 may be modifiedaccording to a response curve having a specific weighting desirable foroperation of LEDs.

FIG. 4 shows an example circuit 45. Circuit 45 transforms the outputfrom isolator 32 to provide a DC output voltage which is relatedexponentially to the input signal (e.g. to the parameter Y illustratedin FIG. 1A). Circuit 45 includes an operational amplifier OA1 having adiode D3 connected in a feedback path. The functional relationshipbetween the input signal and output is determined primarily by theelectrical properties of D3. These properties and the use of diodes infeedback loops are known to those in the art. The output of circuit 45may be provided to a further control 34 or may, in the alternative, beapplied directly to control current through one or more solid-statelight emitters (such as LEDs).

If desired, circuits may be provided to apply offset, switch polarity,amplify or level-shift the signal 31 received through isolator 32.

It is not mandatory that isolator 32 be an opto-isolator. Other forms ofgalvanic isolation may be used for isolator 32. By way of non-limitingexample, isolator 32 may comprise: a transformer, a capacitor, a digitalisolator, a magneto-isolator, an isolation amplifier, a signal transferdevice having a transmitter and receiver that are electrically isolatedfrom one another and exchange signals such as optical, radio, orultrasound signals or the like. In some cases isolator 32 may compriseadditional circuitry to convert signal 31 into a form suitable forpassing through isolator 32. Isolator 32 may provide substantialelectrical and electrical-grounding separation, typically at least 1000volts breakthrough threshold.

FIG. 5 is a block diagram of apparatus 50 according to another exampleembodiment. Apparatus 50 receives AC power 51. The AC power mayoptionally have passed through an AC phase-cut dimmer as describedabove. In FIG. 5, the flow of electrical power is indicated by solidlines while the flow of control signals and information used for controlis indicated by dashed lines.

Incoming AC power 51 optionally passes through an AC filter 52 that isconfigured to remove undesired electrical noise. Filter 52 may, forexample, comprise a low-pass filter. The filtered power is rectified atrectifier 54 which may comprise a full-wave rectifier such as afull-wave bridge, a half bridge or the like.

The rectified power is smoothed by an additional filter 56 and suppliedto the primary side 58A of a SMPS. Power is transferred to secondaryside 58B of the SMPS which supplies electrical current to drive a LEDlight source 60. Current through LED light source 60 is controlled by acurrent control 62.

Apparatus 50 can be seen to have two electrically isolated parts 50A and50B respectively above and below the horizontal line 63. These parts areelectrically isolated from one another and have different groundreferences.

The dimming signal applied by any upstream phase-cut dimmer is taken offat the output of rectifier 54 by a voltage sensor 64 which may comprisea voltage-to-current converter. One such voltage sensor arrangement isillustrated in FIG. 3. The sensed voltage is compared to a referencevalue 66 by a differential comparator 68. The output of differentialcomparator 68 passes from part 50A to part 50B through isolator 70.

The signal is filtered and/or conditioned by suitable circuits 72 andpassed to control 74. In the illustrated embodiment, control 74generates a signal 75A connected to control current control 62, a signal75B connected to control secondary side 58B of the SMPS and a signal 75Cconnected to control LED light source 60.

One or more additional signals are optionally provided to controller 74.Such signals, if present, are carried from part 50A to part 50B by wayof additional isolators (not shown in FIG. 5). Such signals may carryadditional information regarding the waveform of AC power 51 forexample.

FIG. 5 also shows that signal paths may be provided to carry signalsfrom part 50B back to part 50A. In the illustrated embodiment, control74 generates a signal 75D which controls an aspect of the operation ofSMPS primary side 58A. Signal 75D passes from second side 50B to firstside 50A by way of isolator 78. Control 74 may generate signal 75D, forexample, based on information regarding the operational conditions ofone or more of current control 62, light source 60 and SMPS secondary58B and/or information received in one or more signals from part 50A.This architecture can therefore provide an electrically isolatedbi-directional flow of power supply measurement, performance and controldata from both primary to secondary side, and also secondary to primaryside.

A wide range of control schemes may be implemented by control 74. In asimple case, control 74 receives a first signal indicative of a phaseangle of a phase-cut AC waveform and, based on the first signal,generates a second signal that controls the magnitude of a DC currentthrough one or more LED light emitters. In some embodiments generatingthe second signal comprises looking up a value of the first signal in alookup table or calculating a function of the value of the first signal.The second signal may be related to the first signal in a non-linearmanner.

Where the first signal comprises pulses and the phase angle is indicatedby the duty cycle of the pulses then control 74 may determine the dutycycle of incoming pulses by a method comprising: detecting edges of thepulses; calculating the pulse length from difference in time betweenleading and trailing edges; calculating the cycle length from thedifference in time between consecutive leading or trailing edges; andcalculating the duty cycle as a ratio of the pulse length to the cyclelength. The duty cycle may, for example, be expressed as a percentage.Control of the light emitter may be done based on the duty cycle. Thismethod has the advantage of being independent of the frequency of the ACinput signal and will work equally well, for example on 50 Hz or 60 HzAC input. The duty cycle may be mapped to a control output by a suitablefunction (such as a linear or exponential function).

Advantageously the phase angle signal is communicated to control 74 withvery little delay. In the illustrated embodiment, low delay results inpart from the phase angle signal being directly generated by theoperation of an analog circuit. The phase angle signal is generated inreal time and changes in the phase angle are immediately represented inthe phase angle signal. In the illustrated embodiment, the phase anglesignal is directly generated without a separate encoding step, (forexample a step of converting to a series of numbers and thentransmitting the numbers as digital signals).

Low delay facilitates control of the SMPS to achieve optimal efficiencyand/or power factor. For example, the expected power draw from the SMPSmay be determined from the phase angle signal and the SMPS controlled tomake the expected power available. Phase angle and/or othercharacteristics of the input waveform may be monitored and used as abasis for control of the SMPS primary and/or secondary. In someembodiments the SMPS comprises separate, isolated and synchronizedcontrollers for the SMPS primary side and the SMPS secondary side. Suchcontrollers may be configured to maximize power supply performancepertaining to both power factor and efficiency based upon abi-directional flow of information that maintains electrical isolationbetween the SMPS primary side and the SMPS secondary side.

Where the brightness of light source 20 or 50 is being controlled inresponse to the phase angle signal, it may be desirable to apply asmoothing process to prevent large sudden changes in the brightness ofthe light source. Unlike incandescent devices, LEDs have no thermalinertia, an abrupt change in driving an LED results in an abrupt changein the LEDs light output which may be undesirable. Further, some triacphase cut dimmers do not act symmetrically on an AC signal. This canresult in the phase angle signal varying at, for example, 60 Hz. Ifcontrol 74 makes the control of the light emitters track the phase anglesignal then the result may be a flicker in the light delivered.

One approach to preventing sudden changes in light output is to controlthe output based on a running average of the phase angle signal. Forexample, the control 74 may be configured to monitor the phase anglesignal frequently (for example for each cycle or half-cycle of the ACwaveform) and to take a running average of some number of samples (forexample four samples). In an example embodiment, the duty cycle of thephase angle signal is determined and placed into a buffer in a FIFOfashion. The buffer holds N sequential duty cycle values. The contentsof the buffer are summed and the signal applied to control thebrightness of the light emitted by light source 20 is based on the sum.

The result is that any abrupt changes in the phase angle are smoothed.This smoothing also facilitates running the SMPS efficiently. The outputof the SMPS may be controlled to match demand. Smoothing changes in thecommanded brightness of the light emitters can provide time for the SMPSto ramp up to a higher-power mode. Some example methods and apparatusfor controlling a power supply to supply an amount of power based upon acurrent demand are described in US 2008/0224636. Such methods andapparatus may optionally be integrated with the technology describedherein. The rate at which light output is permitted to change may beselected to mimic response characteristics of incandescent lamps.

It is not mandatory to use a programmed processor to provide smoothingof changes in light output. Such smoothing may alternatively be achievedby providing suitable electronic hardware, such as an integratingamplifier, other suitable analog or mixed signal electronic hardware orthe like.

In some embodiments a SMPS has a plurality of separate secondary sidesthat are electrically isolated from one another as well as beingelectrically isolated from a primary side of the SMPS. In some cases itmay be desirable to provide the same signal (such as a phase anglesignal) to controls associated with each of the secondary sides. In someembodiments this is achieved by communicating the same phase angle orother signal into a plurality of electrically isolated domains that arerespectively associated with different SMPS secondary sides by way ofseparate isolators. For example, in a case where a SMPS has first,second and third secondary sides and a single primary side, a phaseangle signal may be generated and passed through first, second and thirdisolators to controllers associated with the first, second and thirdsecondary sides respectively.

FIG. 6 is a block diagram of a light source 80 that illustratesadditional features that may be present in apparatus according to someembodiments of the invention. These features may be combined into otherembodiments (for example those described above). One such feature is theuse of balanced lines to carry signals. A balanced line provides reducedsensitivity to electrical noise. In a balanced line, signals arerepresented as differences in values between two conductors. Inducedpotentials tend to affect both conductors equally and so are rejected.

Another such feature is that some embodiments may provide inputs foradditional or alternative dimmer control inputs. For example, in someapplications dimming control signals are provided by way of low-voltagewiring that is separate from power wiring. In an example embodiment, adimmer signal is a DC voltage in the range of 0 to 10 volts or 1 to 10volts. In such embodiments, one or more isolators may be provided toisolate the part of the circuit that includes the secondary side of theSMPS from the control wiring.

FIG. 6 shows a number of components that are also shown in FIG. 5 theseare identified using the same reference numbers used in FIG. 5. FIG. 6shows a monitoring circuit 81 that measures some characteristic of ACpower 51. For example, monitoring circuit 81 may monitor a phase angleat which half-cycles of an AC waveform are cut by a phase cut dimmer,RMS voltage or power or some other feature of the AC power 51.Monitoring circuit 81 may optionally be configured to also monitor andextract some form of additional information that is embedded within oroverlaid upon the incoming supply AC waveform. The additionalinformation may, for example, signal demand rate change or some otherproperty of AC power 51.

Monitoring circuit 81 generates a signal 82 that is passed through anisolator 83 comprising primary part 83A and secondary part 83B. Signal82 may have any of a wide variety of forms. In some embodiments, signal82 comprises a pulsed signal that recreates the waveform of AC power 51after rectification, an AC signal that has a waveform like that of ACpower 51, a signal having a DC value representing a value of thecharacteristic monitored by monitoring circuit 81 or the like.

Isolator 83 is of a type that can pass signal 82 and may includecircuitry suitable for converting signal 82 into a form that can passthrough the isolating medium of isolator 83.

A balanced line driver 84 is provided at the output on the secondaryside of isolator 83. Balanced line driver 84 drives the two conductorsof a balanced line 85 that carries signal 82 to a receiver 88. Receiver88 extracts signal 82 from the balanced line and provides the signal 82to controller 74.

Controller 74 is also connected to deliver a signal to a SMPS primarycontrol 91 that controls SMPS primary 58A by way of a second balancedline comprising balanced line driver 84A, balanced line 85A and receiver88A and a second isolator 90 comprising primary 90A and secondary 90B.

It can be seen that part 80B of light source 80 is galvanically isolatedfrom part 80A. Isolators 83 and 90 and SMPS 58 all provide galvanicisolation between first part 80A and second part 80B.

In FIG. 6, light source 80 also has a separate control input 92 that maybe connected to receive a separate control signal. Control input 92 may,for example, be configured to receive a low voltage AC or DC analogcontrol signal 95. In alternative embodiments, control input 92 may beconfigured to receive a digital control signal.

Control signal 95 is passed from part 80A to part 80B through isolator93 comprising primary 93A and secondary 93B. In part 80B, signal 95 iscarried by a balanced line transmission path comprising a balanced linedriver 84B, balanced line 85B and receiver 88B. The control signal isdelivered to controller 74.

In some cases it may be desirable to provide control signals tocontroller 74 from a source that is already isolated from AC power 51and from other higher voltages. The control signal may be provided as alow voltage (e.g. 0-10V) signal or a digital signal such as a DigitalAddressable Lighting Interface (“DALI”) signal or the like. FIG. 6 showsan optional direct input 92A for carrying one or more additional controlsignals 95A to controller 74.

Controller 74 may control the brightness of light source 60 based on oneor more of control signals 82 and 95 and 95A. Controller 74 may alsocontrol one or both of the primary and secondary sides of SMPS 58 tomaintain high efficiency at a good power factor based on one or moresignals received from part 80A of light source 80.

In embodiments to be used with triac phase cut dimmers (or other typesof dimmer which require a holding current to be drawn) a holding currentcircuit (not shown) may be provided. In some embodiments the amount ofcurrent drawn by the holding current circuit may be controlled inresponse to the signal measured by monitoring circuit 81 such thatholding current is only drawn when required by a dimmer and in an amountrequired for proper operation of the dimmer.

The embodiments described and illustrated herein are examples only.Features of these embodiments may be combined in other ways than thosedescribed explicitly herein to provide further embodiments. Furthermore,in some applications, certain features illustrated in the exampleembodiments described and illustrated herein may not be required and/oradditional elements may be provided in certain embodiments in ways knownto those of skill in the art in substitution for or in addition toillustrated features. For example, filters are illustrated at variouspoints in the circuits may be replaced with other filter designs inother embodiments and may not be required at all in some embodiments.

Certain implementations of the invention comprise computer processorswhich execute software instructions which cause the processors toperform a method of the invention. For example, one or more processorsin a control as described herein may implement methods ad describedherein by executing software (including firmware) instructions in aprogram memory accessible to the processors. The invention may also beprovided in the form of a program product. The program product maycomprise any medium which carries a set of computer-readable signalscomprising instructions which, when executed by a data processor, causethe data processor to execute a method of the invention. Programproducts according to the invention may be in any of a wide variety offorms. The program product may comprise, for example, non-transitoryphysical media such as magnetic data storage media including floppydiskettes, hard disk drives, optical data storage media including CDROMs, DVDs, electronic data storage media including ROMs, flash RAM, orthe like. The computer-readable signals on the program product mayoptionally be compressed or encrypted.

Where a component (e.g. a software module, processor, assembly, device,circuit, etc.) is referred to above, unless otherwise indicated,reference to that component (including a reference to a “means”) shouldbe interpreted as including as equivalents of that component anycomponent which performs the function of the described component (i.e.,that is functionally equivalent), including components which are notstructurally equivalent to the disclosed structure which performs thefunction in the illustrated exemplary embodiments of the invention.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. Accordingly, the scope of the invention is to beconstrued in accordance with the substance defined by the followingclaims.

What is claimed is:
 1. A power circuit comprising: a first part and asecond part that are galvanically isolated from one another; a switchingmode power supply having a primary side in the first part of the powercircuit and a secondary side in the second part of the power circuit,the primary side connected to receive electrical power from a powerinput, the secondary side connectable to supply electrical power todrive a load comprising one or more solid-state light emitters; aprimary side control in the first part of the power circuit, the primaryside control connected to control operation of the primary side of theswitching mode power supply; a control circuit in the second part of thepower circuit, the control circuit configured to generate a primary sidecontrol signal for the primary side control; a first galvanic isolatorconfigured to carry the primary side control signal from the controlcircuit to the primary side control; a monitoring circuit in the firstpart of the power circuit, the monitoring circuit configured to monitorone or more characteristics of the electrical power from the power inputand to generate an information signal carrying information derived fromthe one or more monitored characteristics; and, a second galvanicisolator configured to carry the information signal from the monitoringcircuit to the control circuit; wherein all of the secondary side of theswitching mode power supply and the control circuit are galvanicallyisolated from the primary side of the switching mode power supply andthe primary side control.
 2. A power circuit according to claim 1wherein: the secondary side of the switching mode power supply is one ofa plurality of secondary sides of the switching mode power supply eachof the plurality of secondary sides connected to provide electricalpower to a corresponding load, each of the loads comprising acorresponding solid-state light source.
 3. A power circuit according toclaim 2 wherein the control circuit is one of a plurality of controlcircuits, each of the control circuits being associated with one of thesecondary sides; each of the plurality of secondary sides of theswitching mode power supply and its corresponding control circuit is ina separate second part of the power circuit; and, each of the separatesecond parts of the power circuit is galvanically isolated from thefirst part of the power circuit; and, the power circuit comprises, foreach one of the secondary sides, a corresponding separate galvanicisolator configured to carry the information signal to the one of thecontrol circuits associated with the secondary side.
 4. A power circuitaccording to claim 3 wherein the plurality of separate second parts ofthe power circuit are electrically isolated from one another.
 5. A powercircuit according to claim 1 comprising a first balanced linetransmission path connected to carry the primary side control signalfrom the control circuit to the first galvanic isolator, the firstbalanced line transmission path comprising a first balanced line driver,a first balanced line and a first receiver.
 6. A power circuit accordingto claim 5 comprising a second balanced line transmission path connectedto carry the information signal from the second galvanic isolator to thecontrol circuit, the second balanced line transmission path comprising asecond balanced line driver, a second balanced line and a secondreceiver.
 7. A power circuit according to claim 1 comprising a controlinput in the first part of the power circuit, the control input separatefrom the power input, the control input connected to provide a controlsignal received at the control input to the control circuit by way of athird galvanic isolator.
 8. A power circuit according to claim 7comprising a third balanced line transmission path connected to carrythe control signal from the third galvanic isolator to the controlcircuit, the third balanced line transmission path comprising a thirdbalanced line driver, a third balanced line and a third receiver.
 9. Apower circuit according to claim 1 wherein the control circuit isconnected to control operation of the secondary side of the switchingmode power supply.
 10. A power circuit according to claim 9 wherein theprimary side control and control circuit are collectively configured tocontrol power factor of the switching mode power supply based on theinformation signal.
 11. A power circuit according to claim 1 wherein thecontrol circuit is configured to generate the primary side controlsignal based on operational conditions of one or more of a currentcontrol, the load, and the secondary side of the switched mode powersupply.
 12. A power circuit according to claim 1 wherein the first andsecond parts of the power circuit have corresponding first and secondground references that are different from one another.
 13. A powercircuit according to claim 1 wherein the monitoring circuit isconfigured so that the information signal comprises pulses having afrequency at least equal to a frequency of an AC waveform of the inputelectrical power and the control circuit is configured to control thesecondary side of the switching mode power supply and has a responsetime not exceeding a period of the pulses.
 14. A power circuit accordingto claim 1 wherein the control circuit comprises a programmable dataprocessor.
 15. A method for controlling a power circuit comprising aswitching mode power supply having a primary side connected to receiveelectrical power from a power input and a secondary side, wherein theprimary side is in a first part of the power circuit, the secondary sideis in a second part of the power circuit and the first part and secondpart of the power circuit are galvanically isolated from one another;the method comprising: in the first part of the power circuit monitoringa characteristic of the electrical power and generating an informationsignal carrying information regarding the monitored characteristic ofelectrical power; passing the information signal to a control circuit inthe second part of the power circuit along a first path comprising afirst galvanic isolator; generating a primary side control signal at thecontrol circuit based at least in part on the information signal;passing the primary side control signal to a primary side controllocated in the first part of the power circuit and connected to controlthe primary side of the switched mode power supply along a second pathcomprising a second galvanic isolator; and, operating the primary sidecontrol to control operation of the primary side based at least in parton the primary side control signal.
 16. A method according to claim 15comprising driving a light source comprising a solid state light emitterwith electrical power output by the secondary side of the switched modepower supply.
 17. A method according to claim 16 comprising controllingoperation of the light source based at least in part on the informationsignal.
 18. A method according to claim 17 wherein the informationsignal carries information specifying a phase angle of the inputelectrical power and the method comprises controlling an output of thelight source based on a running average of the phase angle.
 19. A methodaccording to claim 17 comprising controlling a rate at which lightoutput of the light source is permitted to change to have responsecharacteristics typical of an incandescent lamp.
 20. A method accordingto claim 15 comprising operating the primary side control and thecontrol circuit to control power factor and efficiency of the switchedmode power supply based upon a bi-directional flow of informationbetween the first and second parts of the power circuit whilemaintaining electrical isolation between the first and second parts ofthe power circuit.
 21. A method according to claim 15 wherein monitoringthe characteristic of the electrical power comprises monitoring a phaseangle of the electrical power.
 22. A method according to claim 15comprising maintaining a bi-directional flow of power 5 supplymeasurement, performance and control data between the first and secondparts of the power circuit while maintaining electrical isolationbetween the first and second parts of the power circuit.
 23. A methodaccording to claim 15 wherein generating the information signalcomprises generating pulses having a duty cycle related to a phase angleof the input electrical power.
 24. A method according to claim 23comprising, at the control circuit, detecting edges of the pulses;calculating a pulse length from the difference in time between leadingand trailing edges of the pulses; calculating a cycle length from thedifference in time between consecutive leading or trailing edges of thepulses; and calculating a duty cycle of the pulses as a ratio of thepulse length to the cycle length.
 25. A method according to claim 15comprising controlling both the primary side and the secondary side ofthe switched mode power supply based on the information signal.
 26. Amethod according to claim 25 wherein the switching mode power supplycomprises a plurality of secondary sides and the method comprisescontrolling each of the plurality of secondary sides based on theinformation signal.
 27. A method according to claim 16 whereingenerating the primary side control signal is based on informationregarding the operational conditions of a current control forcontrolling a current to the light source and the light source.